U.S. patent number 8,623,505 [Application Number 13/223,703] was granted by the patent office on 2014-01-07 for decolorizable color developing particle comprising color components present in concentration gradient.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba, Toshiba Tec Kabushiki Kaisha. The grantee listed for this patent is Takeshi Gotanda, Kenji Sano, Yumiko Sekiguchi, Satoshi Takayama. Invention is credited to Takeshi Gotanda, Kenji Sano, Yumiko Sekiguchi, Satoshi Takayama.
United States Patent |
8,623,505 |
Gotanda , et al. |
January 7, 2014 |
Decolorizable color developing particle comprising color components
present in concentration gradient
Abstract
According to one embodiment, decolorizable color developing
particle includes 41 to 50% by mass of a color material relative to
the total amount, while the rest being a binder. The color material
contains an amount m.sub.L of a color developing compound and an
amount m.sub.D (m.sub.D<m.sub.L) of a developer. Island portions
rich in the color material are distributed within sea portion rich
in the binder.
Inventors: |
Gotanda; Takeshi (Yokohama,
JP), Takayama; Satoshi (Kawasaki, JP),
Sekiguchi; Yumiko (Kawasaki, JP), Sano; Kenji
(Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Gotanda; Takeshi
Takayama; Satoshi
Sekiguchi; Yumiko
Sano; Kenji |
Yokohama
Kawasaki
Kawasaki
Tokyo |
N/A
N/A
N/A
N/A |
JP
JP
JP
JP |
|
|
Assignee: |
Kabushiki Kaisha Toshiba
(Tokyo, JP)
Toshiba Tec Kabushiki Kaisha (Tokyo, JP)
|
Family
ID: |
44645589 |
Appl.
No.: |
13/223,703 |
Filed: |
September 1, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20120270975 A1 |
Oct 25, 2012 |
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Foreign Application Priority Data
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Apr 20, 2011 [JP] |
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2011-094299 |
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Current U.S.
Class: |
428/402; 428/218;
428/32.34; 106/31.32; 106/31.3; 106/31.65; 106/31.01 |
Current CPC
Class: |
G03G
9/0926 (20130101); G03G 9/0928 (20130101); B41M
5/305 (20130101); B41M 7/0009 (20130101); Y10T
428/2982 (20150115); Y10T 428/24992 (20150115) |
Current International
Class: |
B32B
5/16 (20060101); B32B 5/00 (20060101); C09D
11/02 (20060101); C09D 7/00 (20060101) |
Field of
Search: |
;503/217,219,225
;106/31.01,31.32,31.6,31.65 ;428/32.34,218,402 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1773382 |
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May 2006 |
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CN |
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0 980 028 |
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Feb 2000 |
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EP |
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1 655 638 |
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May 2006 |
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EP |
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2001-271011 |
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Oct 2001 |
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JP |
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2010-077376 |
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Apr 2010 |
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JP |
|
Other References
European Search Report for European Application No. 11179143.0
dated Jul. 20, 2012. cited by applicant .
Chinese Office Action for Chinese Application No. 201110256333.7
mailed on Jul. 25, 2013. cited by applicant.
|
Primary Examiner: Le; Hoa (Holly)
Attorney, Agent or Firm: Turocy & Watson, LLP
Claims
What is claimed is:
1. A decolorizable color developing particle comprising: a color
material accounting for 41 to 50% by mass of a total amount and
comprising an amount m.sub.L of a color developing compound and an
amount m.sub.D (m.sub.D<m.sub.L) of a developer; and a binder
accounting for a remaining portion, island portions that are rich
in the color material being distributed within a sea portion that
is rich in the binder, wherein when a radius of the color
developing particle is defined as R, and an arbitrary distance from
the center is defined as R.sub.0 (R.sub.0<R), a total area
I.sub.1 of the island portions and an area S.sub.1 of the sea
portion within a region having a radius R.sub.1
(R.sub.1>R.sub.0), and a total area I.sub.2 of the island
portions and an area S.sub.2 of the sea portion within a region
having a radius R.sub.2 (R.sub.2<R.sub.0), satisfy a
relationship shown below:
(I.sub.1/S.sub.1)<(I.sub.2/S.sub.2).
2. The color developing particle according to claim 1, wherein the
amount m.sub.L of the color developing compound exceeds 30 mol %
and 70 mol % or less.
3. The color developing particle according to claim 1, wherein the
amount m.sub.D of the developer is 30 mol % or more and less than
70 mol %.
4. The color developing particle according to claim 1, wherein the
amount m.sub.D of the developer is 0.7 to 0.9 times the amount
m.sub.L of the color developing compound.
5. The color developing particle according to claim 4, wherein the
amount m.sub.D of the developer is 0.75 to 0.8 times the amount mL
of the color developing compound.
6. The color developing particle according to claim 1, wherein the
distance R.sub.o is 30% or less of R, and (I.sub.2/S.sub.2) is 1.1
to 2.3 times (I.sub.1/S.sub.1).
7. The color developing particle according to claim 1, wherein the
distance R.sub.0 is 80% or more of R, and (I.sub.2/S.sub.2) is 1.2
to 4.5 times (I.sub.1/S.sub.1).
8. The color developing particle according to claim 1, wherein the
color developing particle has a glass transition temperature of
87.5.degree. C. or more.
9. The color developing particle according to claim 1, wherein the
color developing compound is Crystal Violet Lactone.
10. The color developing particle according to claim 1, wherein the
developer is 2,4-dihydroxybenzophenone.
11. The color developing particle according to claim 1, wherein the
binder is a styrene/butadiene copolymer.
12. The color developing particle according to claim 1, wherein the
color material accounts for 30 to 70% by mass of a total
amount.
13. The color developing particle according to claim 1, wherein the
color developing particle has an average particle size of 200 to
400 nm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority
from prior Japanese Patent Application No. 2011-094299, filed Apr.
20, 2011, the entire contents of which are incorporated herein by
reference.
FIELD
Embodiments described herein relate generally to a decolorizable
color developing particle.
BACKGROUND
Color developing particles containing a color developing compound
and a developer have been known. These color developing particles
are erasable image forming materials which develop a color when the
level of interaction between the color developing compound and the
developer increases and are decolored when the level of interaction
reduces.
These color developing particles are required to exhibit
sufficiently high color optical density when developing a color.
The color optical density needs to be maintained at a high level
until decoloring is required.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram for explaining a structure of a
decolorizable color developing particle according to an
embodiment;
FIG. 2 is a graphic chart showing a relationship between heating
temperature and coloration retention rate; and
FIG. 3 is a graphic chart showing a relationship between standing
time and coloration retention rate.
DETAILED DESCRIPTION
In general, according to one embodiment, decolorizable color
developing particle includes 41 to 50% by mass of a color material
relative to the total amount, while the rest being a binder. The
color material contains an amount m.sub.L of a color developing
compound and an amount m.sub.D (m.sub.D<m.sub.L) of a developer.
Island portions rich in the color material are distributed within
sea portion rich in the binder.
Hereinafter, embodiments will be specifically described.
The decolorizable color developing particles of the present
embodiment contain a color material, containing a color developing
compound and a developer, and a binder. The amount of the color
material is 41 to 50% by mass relative to the total amount, and the
amount of the developer is lower than the amount of the color
developing compound. Furthermore, the color developing particles of
the present embodiment include sea portion rich in the binder and
island portions rich in the color material that are distributed
within the sea portion.
The present inventors have discovered that the color developing
particles of the present embodiment provided with these conditions
exhibit excellent heat resistance and, as a result, are capable of
retaining the color optical density at a high level.
In the decolorizable color developing particles of the present
embodiment, the island portions rich in the color material are
distributed within the sea portion rich in the binder. Distribution
state of these color developing particles will be explained with
reference to FIG. 1. The radius of a color developing particle is
defined as R, and an arbitrary distance from the center is defined
as R.sub.0. Within a cross-sectional region (radius
R.sub.1>R.sub.0) outside this arbitrary distance R.sub.0, the
total area of the island portions rich in the color material is
defined as I.sub.I, and the area of the sea portion rich in the
binder is defined as S.sub.1. In addition, within a cross-sectional
region (radius R.sub.2<R.sub.0) inside the arbitrary distance
R.sub.0, the total area of the island portions rich in the color
material is defined as I.sub.2, and the area of the sea portion
rich in the binder is defined as S.sub.2.
The following relationship is established between these areas.
(I.sub.1/S.sub.1)<(I.sub.2/S.sub.2)
(I.sub.1/S.sub.1) corresponds to the area ratio of island portions
rich in the color material within a region (having a radius of
R.sub.1) outside the arbitrary distance R.sub.0, and
(I.sub.2/S.sub.2) corresponds to the area ratio of island portions
rich in the color material within a region (having a radius of
R.sub.2) inside the arbitrary distance R.sub.0. The area ratio of
island portions within the inside region is higher than the area
ratio of island portions within the outside region. Throughout the
entire region of the color developing particles according to the
present embodiment, the area ratio of island portions rich in the
color material is higher within the inside region than in the
outside region. It can be said that the composition of the color
developing particles according to the present embodiment has an
inclined structure.
In the color developing particles according to the present
embodiment, the smaller the arbitrary distance R.sub.0, the higher
the ratio of island portions rich in the color material. For
example, within a region where the distance R.sub.0 is about 30% or
less of the radius R of the color developing particles, the
(I.sub.1/S.sub.1) value within a region outside this R.sub.0 is
about 0.4 to 0.6, while the (I.sub.2/S.sub.2) value within a region
inside this R.sub.0 is about 0.7 to 0.9. In this case, the
(I.sub.2/S.sub.2) value is about 1.1 to 2.3 times the
(I.sub.1/S.sub.1) value.
On the other hand, the greater the arbitrary distance R.sub.0, the
higher the ratio of sea portion rich in the binder. For example,
within a region where the distance R.sub.0 is about 80% or more of
the radius R of the color developing particles, the
(I.sub.1/S.sub.1) value within a region outside this R.sub.0 is
about 0.2 to 0.5, while the (I.sub.2/S.sub.2) value within a region
inside this R.sub.0 is about 0.6 to 0.9. In this case, the
(I.sub.2/S.sub.2) value is about 1.2 to 4.5 times the
(I.sub.1/S.sub.1) value.
The area and shape of these island portions and sea portion can be
verified, for example, by using a transmission electron microscope
(TEM) or a scanning electron microscope (SEM). The shape of the
individual island portion in the color developing particles is
generally circular or elliptic. The area per one island portion is
about 0.01 to 5 .mu.m.sup.2.
Each area can be determined, for example, by the following method.
First, the image obtained by the measurement using a TEM or an SEM
at 7,000-times magnification is binarized by expressing the island
portions and the sea portion in two colors (for example, black and
white) using generally available software. Then, the binarized
image is processed using software so that the area for each color
can be determined.
As the color developing compound in the color developing particles
of the present embodiment, for example, electron donating organic
materials such as leucoauramines, diarylphthalides,
polyarylcarbinols, acylauramines, arylauramines, rhodamine B
lactams, indolines, spiropyrans and fluorans can be used.
Specific examples of the color developing compounds include the
following compounds. They are, Crystal Violet Lactone (CVL),
Malachite Green Lactone,
2-anilino-6-(N-cyclohexyl-N-methylamino)-3-methylfluoran,
2-anilino-3-methyl-6-(N-methyl-N-propylamino)fluoran,
3-[4-(4-phenylaminophenyl)aminophenyl]amino-6-methyl-7-chlorofluoran,
2-anilino-6-(N-methyl-N-isobutylamino)-3-methylfluoran,
2-anilino-6-(dibutylamino)-3-methylfluoran,
3-chloro-6-(cyclohexylamino)fluoran,
2-chloro-6-(diethylamino)fluoran,
7-(N,N-dibenzylamino)-3-(N,N-diethylamino)fluoran,
3,6-bis(diethylamino)fluoran-.gamma.-(4'-nitro)anilinolactam,
3-diethylaminobenzo[a]-fluoran,
3-dietylamino-6-methyl-7-aminofluoran,
3-diethylamino-7-xylidinofluoran,
3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azapht-
halide,
3-(4-diethylaminophenyl)-3-(1-ethyl-2-methylindol-3-yl)phthalide,
3-diethylamino-7-chloroanilinofluoran,
3-diethylamino-7,8-benzofluoran,
3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide,
3,6-dimethylethoxyfluoran, 3-diethylamino-6-methoxy-7-aminofluoran,
diethylphosphoromethyl (DEPM), adenosine triphosphate (ATP),
2-(phenylamino)-3-methyl-6-[ethyl(p-tolyl)amino]spiro[9H-xanthen-9,1'(3'H-
)-isobenzofuran-3'-one (ETAC),
2-(2-chloroanilino)-6-dibutylaminofluoran, Crystal Violet Carbinol,
Malachite Green Carbinol, N-(2,3-dichlorophenyl)leucoauramine,
N-benzoylauramine, Rhodamine B lactam, N-acetylauramine,
N-phenylauramine,
2-(phenyliminoethanedilydene)-3,3-dimethylindoline,
N,3,3-trimethylindolinobenzospiropyran,
8'-methoxy-N,3,3-trimethylindolinobenzospiropyran,
3-diethylamino-6-methyl-7-chlorofluoran,
3-diethylamino-7-methoxyfluoran, 3-diethylamino-6-benzyloxyfluoran,
1,2-benzo-6-diethylaminofluoran,
3,6-di-p-toluidino-4,5-dimethylfluoran-phenylhydrazide-.gamma.-lactam,
3-amino-5-methylfluoran, and the like.
As a color developing compound, Crystal Violet Lactone (CVL) is
particularly preferred for its availability and low cost.
As a color developing compound, a single compound may be used
alone, or two or more types of compounds may be used in
combination. When the amount m.sub.L for the color developing
compound within the color material exceeds 30 mol % and about 70
mol % or less, desired effects can be achieved without any
problems. By appropriately selecting the color developing compound,
various colors can be developed and can also be easily adopted in
color applications.
As the developer in the color developing particles of the present
embodiment, for example, phenols, metal phenolates, metal
carboxylates, benzophenones, sulfonic acids, sulfonates, phosphoric
acids, metal phosphates, acidic phosphoric esters, acidic
phosphoric ester metal salts, phosphorous acids, metal phosphites,
and the like can be used.
Specific examples of the developers are listed below. They are,
gallic acid, gallic acid esters such as methyl gallate, ethyl
gallate, n-propyl gallate, propyl gallate, and butyl gallate;
dihydroxybenzoic acids such as 2,3-dihydroxybenzoic acid and methyl
3,5-dihydroxybenzoate, and the esters thereof; hydroxyacetophenones
such as 2,4-dihydroxyacetophenone, 2,5-dihydroxyacetophenone,
2,6-dihydroxyacetophenone, 3,5-dihydroxyacetophenone, and
2,3,4-trihydroxyacetophenone; hydroxybenzophenones such as
2,4-dihydroxybenzophenone, 4,4'-dihydroxybenzophenone,
2,3,4-trihydroxybenzophenone, 2,4,4'-trihydroxybenzophenone,
2,2',4,4'-tetrahydroxybenzophenone, and
2,3,4,4'-tetrahydroxybenzophenone; biphenols such as 2,4'-biphenol
and 4,4'-biphenol; and the like.
In addition, polyhydric phenols such as
4-[(4-hydroxyphenyl)methyl]-1,2,3-benzenetriol,
4-[(3,5-dimethyl-4-hydroxyphenyl)methyl]-1,2,3-benzenetriol,
4,6-bis[(3,5-dimethyl-4-hydroxyphenyl)methyl]-1,2,3-benzenetriol,
4,4'-[1,4-phenylenebis(1-methylethylidene)bis(benzene-1,2,3-triol)],
4,4'-[1,4-phenylenebis(1-methylethylidene)bis(1,2-benzenediol)],
4,4',4''-ethylidenetrisphenol, 4,4'-(1-methylethylidene)bisphenol,
and methylenetris-p-cresol, and the like may also be used.
As a developer, 2,4-dihydroxybenzophenone is particularly preferred
for its availability and low cost.
As a developer, a single compound may be used alone, or two or more
types of compounds may be used in combination. The amount m.sub.D
for the developer within the color material may be from 30 mol % or
more and less than 70 mol %, although the amount m.sub.D for the
developer is smaller than the amount m.sub.L for the color
developing compound. When the amount of the developer is smaller
than the amount of the color developing compound, variations in the
glass transition temperature (Tg) of the obtained color developing
particles may be reduced. For this reason, it becomes possible to
increase the amount of color material containing a color developing
compound and a developer, as compared to the composition with large
developer amount.
However, when the developer amount is too small, the level of
coloration by the color developing compound becomes inadequate. The
amount m.sub.D for the developer is preferably 0.7 to 0.9 times,
and more preferably 0.75 to 0.8 times, as large as the amount
m.sub.L for the color developing compound.
It should be noted that when the glass transition temperature is
reduced to a large extent, the binder reaches a temperature not
less than the glass transition point at a relatively low
temperature. Within this temperature range, low-molecular-weight
components such as the color developing compound and the developer
easily migrate within the binder. When the color developing
particles are applied to toner, the particles are immersed in hot
water. When the particles with a low glass transition temperature
are immersed in hot water, the color developing compound and the
developer easily dissociate, thereby reducing the optical density
of the ultimately obtained toner.
Similar problems to those in the case of toner also arise when the
color developing particles are applied to aqueous ink. Dissociation
of the color developing compound and the developer gradually
proceeds even at room temperature, and it becomes difficult to
maintain the optical density of the color developing particles for
an extended period.
As described above, the color developing particles of the present
embodiment exhibit excellent heat resistance since variations in
the glass transition temperature are small. As a result, it has
become possible to maintain the optical density for an extended
period. Therefore, the color developing particles according to the
present embodiment can also be suitably used for toner and aqueous
ink.
The color material in the color developing particles of the present
embodiment is constituted by the color developing compound and the
developer. The amount of the color material in the color developing
particles is 41 to 50% by mass relative to the total amount. The
remaining portion of the color developing particles is constituted
by the binder. The amount of the color material in the color
developing particles is more preferably 43 to 47% by mass relative
to the total amount.
The amounts of the color developing compound and the developer
contained in the color developing particles can be determined by
gel permeation chromatography (GPC). The binder, the color
developing compound and the developer are dissolved in an eluent,
and the obtained solution is used for the determination. Examples
of the eluents include tetrahydrofuran (THF), chloroform,
dimethylformamide (DMF), dichlorobenzene (DCB), and the like. The
obtained solution is quantified by the GPC method, and each
component can be observed as an individual peak.
In the present embodiment, 3 peaks are mainly observed. In
principle, the greater the molecular weight, the shorter the
retention time. A peak originating from the binder, a peak
originating from the color developing compound and a peak
originating from the developer are detected. For example, the
binder component is observed as a peak having an Mw of 1,000 or
more, whereas the color developing compound and the developer are
observed as peaks having an Mw of 1,000 or less. Note that when
each component is constituted from a plurality of materials, the
number of peaks increase in response to the number of
materials.
In the peak chart obtained by the GPC method, a line connecting the
positions with no peak (i.e., a detected peak) is used as a
baseline. The area for each peak is calculated by using this
baseline as a reference. In this manner, the concentrations of the
color developing compound and the developer contained in the color
developing particles can be determined from the area ratio for the
obtained peaks. When several peaks are overlapping, they are
separated into an individual peak to calculate each area at a
position where the overlapping degree is minimal (a portion serving
as a trough between peaks). It should be noted that the molecular
structure for each peak can be identified from the fragment ion if
the mass spectrum is measured following the separation of each
peak.
The color density increases as the non-polarity of the binder
increases. Examples of the atomic group for increasing the polarity
include an ether group (--O--), a carbonyl group (--C(.dbd.O)--),
an ester group, and the like. The amount of the polar group within
the binder is preferably about 1/3 or less of the molecular
weight.
Examples of such binders include polystyrene, polystyrene
derivatives, copolymers of styrene, and the like. These binders can
be obtained, for example, by polymerizing a styrene-based monomer
selected from the group consisting of styrene, o-methylstyrene,
m-methylstyrene, p-methylstyrene, p-ethylstyrene,
2,4-dimethylstyrene, p-n-butylstyrene, p-tert-butylstyrene,
p-n-hexylstyrene, p-n-octylstyrene, p-n-nonylstyrene,
p-n-decylstyrene, p-n-dodecylstyrene, p-methoxystyrene,
p-phenylstyrene, p-chlorostyrene and 3,4-dichlorostyrene.
Examples of copolymers of styrene include styrene/butadiene
copolymers, styrene/p-chlorostyrene copolymers, styrene/propylene
copolymers, styrene/butadiene rubbers, and the like.
As the binder, styrene/butadiene copolymers are particularly
preferred due to their high thermal stability.
The color developing particles of the present embodiment can be
produced, for example, by the following method. First, the color
developing compound, the developer and the binder are added to a
solvent to prepare a solution. The solvent can be selected from,
for example, toluene, hexane, acetone, or the like. This solution
is sprayed into a gaseous phase to form droplets. The solvent is
separated from the obtained droplets to form the decolorizable
color developing particles, and the color developing particles are
then collected. The solvent is evaporated and separated from the
free surface of the droplets.
Although there are no particular limitations on the gas species in
the gaseous phase, the oxygen concentration is preferably 5% or
less in order to avoid the risk of ignition or explosion of the
droplets. With respect to the gas species, a nonflammable gas or a
rare gas is preferred. Specific examples thereof include nitrogen
gas, carbon dioxide, rare gas, helium gas, neon gas, argon gas, and
the like. These gas species can be used alone, or two or more types
thereof may be mixed for use.
There are no particular restrictions on the spraying method.
However, more specifically, a two-fluid nozzle, a one fluid nozzle,
an ultrasonic nozzle, a piezoelectric nozzle, a thermal head-type
nozzle, an electrostatic spraying nozzle, and the like can be
employed. A two-fluid nozzle and an electrostatic spraying nozzle
are particularly suitable because the size of the produced
particles is small.
Drying and color development proceed at the same time inside the
droplets immediately after spraying, and the solvent evaporates
from the free surface of the droplets. At this time, the binder
which is a slowly diffusing resin component is concentrated outside
the droplets. On the other hand, the color developing compound and
the developer that diffuse rapidly migrate to the inside of the
droplets. In the color developing particles obtained following
drying, the number of island portions rich in the color material
containing a color developing compound and a developer increases on
the inner side so as to form an inclined structure.
The color developing particles of the present embodiment preferably
have a glass transition temperature of 87.5.degree. C. or more. The
color development level of these color developing particles can be
maintained when the temperature is not less than the above
temperature. The glass transition temperature of the color
developing particles can be measured, for example, by differential
scanning calorimetry (DSC) or the like. Note that the rate of
temperature increase in the case of the measurement by DSC is
10.degree. C./min.
Specific examples of the decolorizable color developing particles
are shown below.
The color material containing the color developing compound and the
developer, as well as the binder were dissolved in a solvent in
accordance with the formulations indicated in Table 1 shown below
to obtain solutions Nos. 1 to 6. The mol % for the color developing
compound and the developer refers to the percentage with respect to
the number of moles within the color material as a whole. The
amount of the dissolved color material (parts by mass) corresponds
to the total amount (% by mass) of the color material in the
ultimately obtained color developing particles. All solutions were
prepared by dissolving the components so that the total amount
thereof is 1.25 g/100 ml.
TABLE-US-00001 TABLE 1 1 2 3 4 5 6 Color CVL CVL CVL CVL CVL CVL
developing compound Developer 2,4- EG EG EG EG EG DHBP Color
material 30 41 45 50 70 50 (parts by mass) Binder 70 59 55 50 30 50
(parts by mass) Color 70 55 60 60 60 30 developing compound
m.sub.L(mol %) Developer 30 45 40 60 40 70 m.sub.D(mol %)
m.sub.D/m.sub.L 0.429 0.818 0.667 0.667 0.667 2.333
The used materials are summarized below. Color developing compound:
Crystal Violet Lactone CVL (leuco dye manufactured by Yamada Kagaku
Co., Ltd.) Developer: 2,4-dihydroxybenzophenone (2,4-DHBP) Ethyl
gallate (EG) Binder: polystyrene (brand G320C manufactured by Toyo
Styrol Co., Ltd.) Solvent: a mixed solvent of acetone (70% by mass)
and toluene (30% by mass)
Each solution was sprayed in a nitrogen atmosphere using a spray
dryer (B-290 type, manufactured by Sibata Scientific Technology
Ltd.) to obtain color developing particles Nos. 1 to 6. The ambient
temperature where the spraying was conducted was controlled between
55 and 60.degree. C. by external heating. The temperature control
was conducted using a heater. The average particle size for the
obtained color developing particles was about 200 to 400 nm when
determined by a particle size distribution measuring apparatus. The
radius R for the color developing particles was about 100 to 200
nm.
When the obtained color developing particles were observed using a
TEM, in the color developing particles Nos. 2 to 6, it was
confirmed that the island portions rich in the color material were
are distributed within the sea portion rich in the binder.
With respect to each color developing particle, the distance
R.sub.0 was set to 50% of the radius R, so as to define an outside
region and an inside region, as shown in FIG. 1. Area I.sub.1 of
the island portions and area S.sub.1 of the sea portion in the
outside region were determined from the image obtained from the TEM
to calculate the area ratio (I.sub.1/S.sub.1) in the outside
region. With respect to the inside region, area I.sub.2 of the
island portions and area S.sub.2 of the sea portion were determined
in the same manner to calculate the area ratio (I.sub.2/S.sub.2) in
the inside region.
Moreover, Tg of each color developing particle was measured by DSC.
The results are summarized in Table 2 shown below together with the
area ratio.
TABLE-US-00002 TABLE 2 1 2 3 4 5 6 I.sub.1/S.sub.1 -- 0.01 0.1 0.1
0.3 0.4 I.sub.2/S.sub.2 -- 0.5 0.7 0.7 0.8 0.5 Tg 60 87.5 88 89 70
60
As shown in the Table 2 above, in the color developing particles
Nos. 2 to 6, it was confirmed that the area ratio for the island
portions was higher in the inside region than in the outside
region, although the presence of island portions was not confirmed
in the color developing particles, No. 1. Tg for the color
developing particles Nos. 2 to 4 was as high as 87.5.degree. C. or
even higher. In these particles, the total amount of the color
material in the color developing particle is within a range from 41
to 50% by mass, and the amount of the developer is more than the
amount of the color developing compound. In those cases where the
total amount of the color material in the color developing
particles was 30% by mass (No. 1) and 70% by mass (No. 5), Tg was
60 and 70.degree. C., respectively.
When the amount of the developer is more than the amount of the
color developing compound, it has been shown in the example of No.
6 that Tg is 60.degree. C. even if the total amount of the color
material in the color developing particles is 50% by mass.
Then, the color optical density of each color developing particle
was measured using a colorimeter (manufactured by Konica Minolta
Holdings, Inc.). It is required that the color optical density be
0.5 or more.
Moreover, an accelerated test was conducted to examine the
coloration retention of each color developing particle. Each color
developing particle was dispersed in Vylonal MD-1200 (Toyobo Co.,
Ltd.), and then immersed in water at 70.degree. C. for 15 minutes.
Vylonal is used as a dispersing agent. Initial optical density was
measured for each color developing particle and defined as the
optical density (D.sub.0) before fading. In addition, the optical
density following the treatment was measured and defined as the
optical density (D.sub.1) after fading. The coloration retention
rate was calculated from the formula: 100.times.(D)/(D.sub.0).
The results are summarized in Table 3 shown below together with the
color optical density. It is required that the coloration retention
rate be 60% or more.
TABLE-US-00003 TABLE 3 1 2 3 4 5 6 Color optical density 0.3 0.7
0.6 0.7 0.75 0.5 Coloration retention rate (%) 10 61 70 75 30
10
As shown in the Table 3 above, the color developing particles Nos.
2 to 4 exhibited excellent properties in terms of both the color
optical density and the coloration retention rate. In the color
developing particles Nos. 2 to 4, the total amount of the color
material containing the color developing compound and the developer
is within a range from 41 to 50% by mass, and the amount of the
developer is less than the amount of the color developing
compound.
In the case of the particles No. 1 where the island portions rich
in the color material were not clearly observed, the color optical
density was as low as 0.3, and the coloration retention rate was
also as low as 10%. Moreover, the glass transition temperature of
the color developing particle No. 1 is as low as 60.degree. C.
The coloration retention rates of the color developing particles
No. 5 and No. 6 were as low as 30% and 10%, respectively. The cause
for this observation is that the total amount of the color material
was as large as 70% by mass in the color developing particles No. 5
and the amount of the developer is more than the amount of the
color developing compound in the color developing particles No.
6.
As shown in the graph of FIG. 2, the coloration retention rate
tends to reduce as the heating temperature increases. Note that the
heating temperature herein refers to a liquid temperature. A
coloration retention rate of 60% or more can be ensured as long as
the heating temperature is as high as the glass transition
temperature (Tg: 87.5.degree. C.)
In addition, as shown in the graph of FIG. 3, the coloration
retention rate tends to reduce at an early stage after being left
to stand. This tendency of reduction is dependent on the ambient
temperature, and a coloration retention rate of 70% or more can be
ensured as long as the ambient temperature does not exceed the Tg
value. Even if exposed to a high temperature condition of
80.degree. C. for 2 hours, it has been shown that the coloration
retention rate tends to become substantially constant without
further reduction. Therefore, even under room temperature
conditions, it has been expected that the level of coloration can
be retained satisfactorily.
The decolorizable color developing particles of the present
embodiment are capable of retaining the color optical density at a
high level in a favorable manner.
While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to
limit the scope of the inventions. Indeed, the novel embodiments
described herein may be embodied in a variety of other forms;
furthermore, various omissions, substitutions and changes in the
form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
* * * * *